If I want to sell a coin other than bitcoin and I attempt to exchange the coin for specific amount of BTC. After creating the request to exchange the coin for btc, Are they sold immediately? Or I have to wait till I get a buyer?

PS: I'm a bounty hunter so This post equally seeks for ways to exchange/sell bounty rewards for BTC If the coin are listed in exchange.

In a simple sentence, I want to know the process of exchanging altcoin/bitcoin

There will be no big differences from your local shop to a bitcoin/altcoin exchange. They are virtually supporting peer to peer exchanges. (very few exchanges themselves populate some orders to fill the gaps).

The process of exchanging bitcoin/altcoin is literally similar to a barter system but good for goods. One significant differences : there will be some fees to exchanges.